Recently, mobile communications devices such as cellular phones have been progressively spread due to the development of information-oriented society. Acoustic wave devices are used in the mobile communications devices as duplexers, transmission bandpass filters or reception bandpass filters, and are required to be downsized, weight-lightened and more reliable.
A surface acoustic wave device composed of an IDT and reflectors formed on a piezoelectric substrate is a typical example of the acoustic wave devices. An acoustic wave is excited by an electric signal applied to the IDT and a resultant electric signal of a frequency in the pass band is output.
Now, a SAW device described in Japanese Patent No. 3405329 (Document 1) is described below. Document 1 describes a technique of providing a reaction restraining film and a metal layer on an interconnection electrode.
FIG. 1A is a plan view of a SAW device 100. Reflectors 4 and IDTs 6 made of a metal such as aluminum are formed on a piezoelectric substrate 2 made of lithium niobate (LiNbO3) or lithium tantalate (LiTaO3). Further, there are provided interconnections 10 connected to the IDTs 6 and external connection terminals 8 connected to the interconnections 10.
FIG. 1B is a cross-sectional view taken along a line A-A depicted in FIG. 1A. Referring to FIG. 1B, each of the interconnections 10 is composed of an interconnection electrode 12, a first metal layer 14, a second metal layer 16, a third metal layer 18, and an inorganic insulation layer 20. The interconnection electrode 12 is formed on the piezoelectric substrate 2 and may be made of aluminum. The first metal layer 14 is provided on the interconnection electrode 12 and may be made of titanium. The second metal layer 16 is provided on the first metal layer 14 and may be made of palladium. The inorganic insulation layer 20 is interposed between the interconnection electrode 12 and the first metal layer 14.
FIG. 2A illustrates a variation of the SAW device illustrated in FIG. 1B. The inorganic insulation layer 20 is provided so as to cover the side surfaces of the interconnection electrode 12. Document 1 describes that reaction of the interconnection electrode 12 with the metal layers can be restrained, so that the junctions between the interconnection electrode 12 and the metal layers and the junctions between the interconnections 10 and the external connection terminals 8 can be strengthened.
However, in the structure illustrated in FIG. 1B, since side surfaces 12a of the interconnection electrode 12 are exposed, the interconnections 10 do not have good moisture resistance. Moisture that enters into the interconnection electrode 12 corrodes it and degrades the reliability of the SAW device.
FIG. 2B is an enlarged view of one of the side surfaces 12a of the interconnection electrode 12 and its vicinity. Referring to FIG. 2B, there is a great step between the piezoelectric substrate 2 and the interconnection electrode 12, and the thickness of the inorganic insulation layer 20 may be reduced unwillingly (backward tapered). In this case, the interconnection electrode does not have good moisture resistance, and therefore, the reliability of the SAW device may be degraded.